by Din » Thu Sep 06, 2018 8:32 am
lobaz wrote: ↑Wed Sep 05, 2018 5:15 pm
Thus, the hologram does a sort of "inverse" to the scattering - it is able to cancel it out.
It's basically phase conjugation.
Let's say you pass light through a scattering medium, say a cube of clear jelly. Let's illuminate the cube of jelly with an unexpanded laser beam and follow two rays of light through the jelly. One ray is scattered through, say, 30 degrees to the beam axis and another is scattered through -15 degrees, ie it's on the other side of the beam axis. Now, if you put a mirror behind the cube and parallel to it, the light that scattered at 30 deg, will reflect at 30 away from the cube and similarly for the -15 deg light. But now, let's again pass light through the cube, but this time place a holographic plate behind the cube. Reference the hologram from the other side of the plate, ie the side away from the cube. Now, once again, pass a laser beam through the cube. Again, one ray is scattered through 30 and one through -15. But, now, you're holographically recording the two rays of light, with a separate reference beam behind the plate, ie you're recording in transmission geometry. If you now reconstruct the hologram, the reconstructed beams will go back along the same 30 deg and -15 deg vectors. The hologram will act as a 'phase conjugate mirror', in that it will exactly reverse the direction of the light back to where it originally came from, because that's what you recorded in the first place. If you now replace the cube, the reconstructed rays of 30 and -15 re-enter the cube. If the properties of the cube have not changed, the ray entering at -15, and the ray entering at 30, will both leave the cube in the same direction as the originally illuminating laser beam.
lobaz wrote: ↑Wed Sep 05, 2018 5:15 pm
Thus, the hologram does a sort of "inverse" to the scattering - it is able to cancel it out.
It's not the hologram that inverts the scattering, the hologram just records the wavefunction of the scattered light. By passing the reconstructed wavefunction through the same medium, the medium reverses the scattering, by the Principle of Reversibility, which says that any light propagating in one direction, can be propagated in the reverse direction in exactly the same way. If you placed a different cube in the path of the reconstructed wavefront, the illuminating light would not emerge, the light would not be 'unscattered'.
[quote=lobaz post_id=69744 time=1536185716 user_id=2217]
Thus, the hologram does a sort of "inverse" to the scattering - it is able to cancel it out.
[/quote]
It's basically phase conjugation.
Let's say you pass light through a scattering medium, say a cube of clear jelly. Let's illuminate the cube of jelly with an unexpanded laser beam and follow two rays of light through the jelly. One ray is scattered through, say, 30 degrees to the beam axis and another is scattered through -15 degrees, ie it's on the other side of the beam axis. Now, if you put a mirror behind the cube and parallel to it, the light that scattered at 30 deg, will reflect at 30 away from the cube and similarly for the -15 deg light. But now, let's again pass light through the cube, but this time place a holographic plate behind the cube. Reference the hologram from the other side of the plate, ie the side away from the cube. Now, once again, pass a laser beam through the cube. Again, one ray is scattered through 30 and one through -15. But, now, you're holographically recording the two rays of light, with a separate reference beam behind the plate, ie you're recording in transmission geometry. If you now reconstruct the hologram, the reconstructed beams will go back along the same 30 deg and -15 deg vectors. The hologram will act as a 'phase conjugate mirror', in that it will exactly reverse the direction of the light back to where it originally came from, because that's what you recorded in the first place. If you now replace the cube, the reconstructed rays of 30 and -15 re-enter the cube. If the properties of the cube have not changed, the ray entering at -15, and the ray entering at 30, will both leave the cube in the same direction as the originally illuminating laser beam.
[quote=lobaz post_id=69744 time=1536185716 user_id=2217]
Thus, the hologram does a sort of "inverse" to the scattering - it is able to cancel it out.
[/quote]
It's not the hologram that inverts the scattering, the hologram just records the wavefunction of the scattered light. By passing the reconstructed wavefunction through the same medium, the medium reverses the scattering, by the Principle of Reversibility, which says that any light propagating in one direction, can be propagated in the reverse direction in exactly the same way. If you placed a different cube in the path of the reconstructed wavefront, the illuminating light would not emerge, the light would not be 'unscattered'.